Have you ever wondered where they store the oxygen for the emergency masks aboard airplanes? It seems like every day, airlines are finding more ways to save money and fill the plane to its absolute capacity. Considering that, you'd think it keeping heavy, bulky oxygen tanks onboard would be pretty impractical. And it is, especially when there's another way: create your own oxygen.

Attention All Passengers

These days, most airlines charge for checked bags, and many even charge for carry-on bags. Add in the price of each seat, and you can see how airlines have an incentive to get as many passengers and bags into the plane as possible. That means finding ways to make necessary equipment smaller and lighter. Considering the fact that oxygen tanks, like those used by SCUBA divers, can each weigh as much as a full suitcase, it's clear that they're not the best option for emergency oxygen.

So airlines use small, tennis-ball-can sized canisters known as oxygen candles — and despite the name, they don't contain oxygen. Not at first, anyway. Instead, they rely on a nifty chemical reaction that actually frees pure oxygen from other compounds.

The Magic Reaction

There are plenty of non-gas compounds that are rich in oxygen. Take a look at the O3 in the chemical formula of sodium chlorate, NaClO3, for example. All it needs is a little bit of heat, and sodium chlorate quickly gives off its oxygen so that you have air to breathe. Same goes for the chemicals barium peroxide and potassium perchlorate.

That's why flight attendants tell you to tug on the mask. That tug creates a small explosion that generates the heat it takes to make the compound release its oxygen. The chemical reaction only lasts for a maximum of about 20 minutes, but that's usually enough time for the plane to descend to a safe breathing altitude. Still, it's extremely important that you put your mask on as soon as possible — it only takes about 30 seconds in dangerously low cabin pressure for a person to pass out.

For more answers to those nagging airline questions, check out "Full Upright and Locked Position: The Insider's Guide to Air Travel" by former FAA chief counsel and senior aviation policy official Mark Gerchick. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.